Medium to high voltage semiconductor devices such as gate turnoff thyristors (GTOs), silicon controlled rectifiers (SCRs) and insulated gate bipolar transistors (IGBTs) are used in a variety of power system applications. For example, IGBTs may be used as the switching elements in a power inverter bridge controlling a 1200 horsepower motor. These medium to high power semiconductor devices are characterized by current and voltage ratings of approximately 100 to 300 amps and 1.2 to 10 kV.
Due to the relatively high power capacities of such semiconductor devices they have to be packaged so as to contend with a number of issues, including heat dissipation, electrical contact characteristics and arcing. One common form of packaging used by the manufacturers of such devices is the "press pack". In this packaging structure the semiconductor material is enclosed in a typically cylindrically-shaped casing. The tubular body of the casing is constructed out of a non-electrically conductive material; ceramic is often used for its durability at high temperatures. The tubular body is capped with electrically conductive metallic plates which function as some (or all) of the terminals of the semiconductor device, such as the anode and cathode of a GTO or thyristor. These terminal end-faces present relatively broad planar surfaces for enabling good electrical and thermal contact with other power circuit components such as electrically conductive heat sinks. To further ensure good electrical contact and meet other operating requirements, press pack devices require a pre-specified amount of pressure to be applied thereto, typically in the range of 2-20 kN, although much higher forces are also possible.
The pressure or mounting force applied to the press-pack devices is provided by some sort of clamping mechanism. A typical clamping mechanism comprises two threaded rods fitted with plates for applying pressure provided by clamping nuts. Vice-like clamping mechanisms can also be used. These clamping mechanism are also often used to stack multiple numbers of press-pack devices and heat sinks together in abutting relationship. The resultant assembly, or "stack", can be used in a variety of power circuits such as the leg of an inverter and minimizes the number of clamping mechanisms required, which are extraneous elements of the power circuit.
In assembling the stack the conventional practice is to axially align all of the elements thereof in order to ensure uniform application of the mounting force. The way this was accomplished in the prior art is through the use of small guide pins inserted into locating holes formed on the abutting faces of the press-pack devices and heat sinks. Many press-pack devices are manufactured with small holes situated in the centre of the terminal end-faces thereof for this purpose. However, a significant problem arises with this system when it is necessary to replace one of the press-pack devices in the field. More specifically, the heat sinks of the stack are typically quasi-rigidly mounted to a supporting structure such as a housing or cabinet and therefore capable of moving apart only a few thousands of an inch to allow for thermal expansion. This distance is considerably less than the length of the guide pin as disposed in the locating hole. So, to replace one press-pack device in a large stack often meant the whole stack had to be removed from the cabinet, disassembled to replace the press-pack device, then re-assembled and re-installed. This task could require well over an hour. Alternatively, field personnel would attempt to bypass the disassembly procedure altogether by trying to pry out a press-pack device from the stack through the use of sheer force. This usually resulted in a significant scarring or gouging of the terminal end-faces of the press-pack device caused by dragging it over the embedded locating pins, and the gouges were often significant enough so as to render the press-pack devices inoperative because of a change in the thermal transfer characteristics.